If you mean the shift takes some time after you shift the shift lever, that's not the shift lever's issue. Your best bet would be to check your RD alignment, replace shift cables & housings, and adjust the derailleurs. Also, the rear downshifts occur at the shift ramps on the cassette cogs, so they're still the final bottleneck. You have to wait for them to come around.

If the issue is that the shift doesn't occur until you pull AND RELEASE the upshift lever, then you could get XTR M970 shift pods, which not only feature instant release, but can upshift two cogs in one shot if you pull the upshift lever past the first detent. Nice shifters (but not cheap).